A lot of formalin-inactivated Japanese B encephalitis vaccine was prepared from an attenuated strain of OCT-541 virus, grown in hamster kidney cell, in medium 199 and 2% human albumin at pH 7.9. The pre-inactivation infectivity and hemagglutination titers were 108.3/0.1 ml and 1:256 respectively after Millipore filtrations. The virus suspension was inactivated with 1:4000 formalin at 37°C for three times the period estimated for destruction of measurable infectivity. Sterility and safety tests showed that the final vaccine was free from bacteria and other detectable adventitious agents. The potency of the vaccine was established by the mouse test (minimal immunogenic dose of 0.0089 ml) and by the development of high anti-body titer in immunized guinea pigs and monkeys (neutralization index of >4.8 logs). One vaccinated monkey observed for 9 months maintained a high antibody level for this period.
Data regarding the vaccine stability were presented for other lots with longer storage, which suggested that freezing did not lower the potency and the one preparation frozen at about -70°C and observed for about 14 months maintained its potency for that period.